Ouch! Well, this goes in a file marked "extreme" for me (It was also an SE7.8 which seems high for an Advanced )
I did throw the kitchen sink at it - but still got nowhere. Finally the "fish" hint steered me towards x-chains and this was my best path in the end (after more than an hour)...

Peter, I don't know what an X-Chain is, although it appears to involve just one number. Maybe an advanced form of Simple Coloring?

At any rate, as you know, I'm notation-challenged and can't fully follow your first chain, but I can follow it from the grid. Basically, I think, if r3c1<>5, then r46c2=5, thus allowing the eliminations.

That's a very nice move, but how did you find that little isle in this ocean of poly-value cells?

Danny, I even put it into Hoduko after to see if I had been stupid - you should see the mess that makes of the solution path by not playing the chain on 5s!!

Marty R. wrote:

That's a very nice move, but how did you find that little isle in this ocean of poly-value cells?

Marty, I use Simple Sudoku as my helper. I used to do simple and multi-colouring - this x-chain on 5 would need three levels of colouring though. Now I tend to look directly for x-chains/turbots. I filter on the candidate and then pick a strong link, look to see if the end can see another strong link, repeat, check for eliminations etc. Easy if using SS, not so on paper...